1. Field of the Invention
This invention may be classified as one relating to the preparation and formulation of polyimide polymers and copolymers.
2. Description of Prior Art
Composite materials have been used extensively as structural materials in aerospace and other applications where high strength, light weight materials capable of withstanding high temperatures are required. Considerable efforts to extend the thermal stability range, while retaining good structural strength without increasing the weight of such materials has been expanded. Currently, addition polymers such as epoxy resins are used in conjunction with fibers or fabrics to provide essentially void free composite structures which exhibit good structural properties and are light in weight. These structures, however, are limited in their use temperatures to about 150.degree.-175.degree. C because of the thermal stability characteristics of expoxy resins.
There are polyimides such as "P13N" from Ciba Geigy Corporation which give very low void content laminates which are useful at temperatures up to 288.degree. C and there is a polyimide known as Kerimid 601 from Rhodia Corporation (a subsidiary of Rhone Poulenc Co.) which can provide void free laminates which can withstand temperatures of up to 260.degree. C.
Higher temperature laminating resins which cure through addition were unknown prior to the development of the polyimides No. described in the Hughes Aircraft Company U.S. Pat. No. 3,845,018, although there are condensation type polyimides which can be used to produce laminates which withstand temperatures up to 300.degree.-320.degree. C. These resins are limited in their usefulness because the laminates and/or composite materials produced from them exhibit void contents as high as 20-40%. The voids are primarily caused by outgassing which occurs during the condensation mechanism cure.
Applicant herein, in conjunction with Drs. A. J. Landis and L. J. Miller of Hughes Aircraft Company, developed acetylene substituted polyimide oligomers which cure through addition rather than condensation in an attempt to solve the void problem discussed above and retain good thermal resistant properties. Polyimide composites with as little as several tenths to 1% voids were obtained even when molding pressures as low as 200 psi were used. These materials are described in U.S. Pat. No. 3,845,018.
Applicant's present invention constitutes a substantial improvement over the invention described in U.S. Pat. No. 3,845,018 as well as the prior art in that it facilitates the preparation of laminates and composites having high thermal stability, zero void content, and better mechanical properties because of the lower viscosity imparted by the incorporation of the reactive diluent. This is especially true in applications where inpregnation of the oligomer into a highly porous substrate is important.